Match PremiumMoney interface to Money
Round premium payment amounts using bankers' rounding
Implement bankers' rounding manually

The Ruby 1.9.2 BigDecimal implementation contains a bug affecting rounding half to even:
http://bugs.ruby-lang.org/issues/show/3803
Merge pull request #123 from alphagov/bankers_rounding

Use bankers' rounding on premium payment amounts
Revert "Merge pull request #123 from alphagov/bankers_rounding"

This reverts commit 7ac0b2b4aa41146837a53959bdd34c17c4baba76, reversing
changes made to e084bb5adb2cfa6b4563471f75a43f84efbda6b7.

The merged code broke the build, so undoing will ask for a fix on the
relevant branch.